Using slots class overrides custom `__setattr__` in 20.1.0

[pganssle]

In 19.3.0, it was possible to have a custom __setattr__ on your slots class, but in 20.1.0 the custom __setattr__ is replaced with the default one:

import attr
@attr.s(slots=True)
class A:
    def __setattr__(self, key, value):
        print(f"{key}: {value}")

In 19.3.0:

>>> A().a = 3
a: 3

In 20.1.0:

>>> A().a = 3
AttributeError: 'A' object has no attribute 'a'
>>> A.__setattr__
<slot wrapper '__setattr__' of 'object' objects>

When slots=False, the same thing does not occur.

[hynek]

Now this is very embarrassing. Does #681 look good to you?